Manganese Catalysts with Molecular Recognition Functionality for Selective Alkene Epoxidation
ABSTRACT Selective epoxidation of alkenes is possible with a new manganese porphyrin catalyst, C(PMR), that uses hydrogen bonding between the carboxylic acid on the substrate molecule and a Kemp's triacid unit. For two out of three olefin substrates employed, molecular recognition prevents the unselective oxidation of C-H bonds, and directs oxidation to the olefin moiety, giving only epoxide products. Weak diastereoselectivity is observed in the epoxide products, suggesting that molecular recognition affects the orientation of the catalyst-bound substrate. The previously reported manganese terpyridine complex C(TMR) is shown to be a superior epoxidation catalyst to the porphyrin catalyst C(PMR). Good conversion of 2-cyclopentene acetic acid (substrate S2) with C(PMR) is consistent with molecular modeling, which indicates a particularly good substrate/catalyst match. Evidence suggests that hydrogen bonding between the substrate and the catalyst is critical in this system.
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ABSTRACT: New catalysts for non-directed hydrocarbon functionalization have great potential in organic synthesis. We hypothesized that incorporating a Mn-terpyridine cofactor into a protein scaffold would lead to artificial metalloenzymes (ArMs) in which the selectivity of the Mn cofactor could be controlled by the protein scaffold. We designed and synthesized a maleimide-substituted Mn-terpyridine cofactor and demonstrated that this cofactor could be incorporated into two different scaffold proteins to generate the desired ArMs. The structure and reactivity of one of these ArMs was explored, and the broad oxygenation capability of the Mn-terpyridine catalyst was maintained, providing a robust platform for optimization of ArMs for selective hydrocarbon functionalization.Tetrahedron 03/2014; Available online 12 March 2014(27-28). DOI:10.1016/j.tet.2014.03.008 · 2.82 Impact Factor
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ABSTRACT: We report a diastereoselective synthetic method to obtain a family of catalytic molecular baskets containing a spacious cavity (~570 Å(3)). These supramolecular catalysts were envisioned, via the process of gating, to control the access of substrates to the embedded catalytic center and thereby modulate the outcome of chemical reactions. In particular, gated basket 1 comprises a porphyrin "floor" fused to four phthalimide "side walls" each carrying a revolving aromatic "gate". With the assistance of (1)H NMR and UV-vis spectroscopy, we demonstrated that the small 1-methylimidazole guest (12, 94 Å(3)) would coordinate to the interior while the larger 1,5-diadamantylimidazole guest (14, 361 Å(3)) is relegated to the exterior of basket Zn(II)-1. Subsequently, we examined the epoxidation of differently sized and shaped alkenes 18-21 with catalytic baskets 12(in)-Mn(III)-1 and 14(out)-Mn(III)-1 in the presence of the sacrificial oxidant iodosylarene. The epoxidation of cis-stilbene occurred in the cavity of 14(out)-Mn(III)-1 and at the outer face of 12(in)-Mn(III)-1 with the stereoselectivity of the two transformations being somewhat different. Importantly, catalytic basket 14(out)-Mn(III)-1 was capable of kinetically resolving an equimolar mixture of cis-2-octene 20 and cis-cyclooctene 21 via promotion of the transformation in its cavity.The Journal of Organic Chemistry 02/2012; 77(6):2675-88. DOI:10.1021/jo202443j · 4.64 Impact Factor
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ABSTRACT: A versatile class of heme monoxygenases involved in many vital functions for human health are the cytochromes P450, which react via a high-valent iron(IV) oxo heme cation radical species called Compound I. One of the key reactions catalyzed by these enzymes is C═C epoxidation of substrates. We report here a systematic study into the intrinsic chemical properties of substrate and oxidant that affect reactivity patterns. To this end, we investigated the effect of styrene and para-substituted styrene epoxidation by Compound I models with either an anionic (chloride) or neutral (acetonitrile) axial ligand. We show, for the first time, that the activation enthalpy of the reaction is determined by the ionization potential of the substrate, the electron affinity of the oxidant, and the strength of the newly formed C-O bond (approximated by the bond dissociation energy, BDEOH). We have set up a new valence bond model that enables us to generalize substrate epoxidation reactions by iron(IV)-oxo porphyrin cation-radical oxidants and make predictions of rate constants and reactivities. We show here that electron-withdrawing substituents lead to early transition states, whereas electron-donating groups on the olefin substrate give late transition states. This affects the barrier heights in such a way that electron-withdrawing substituents correlate the barrier height with BDEOH, while the electron affinity of the oxidant is proportional to the barrier height for substrates with electron-donating substituents.Inorganic Chemistry 07/2013; 52(14). DOI:10.1021/ic4005104 · 4.79 Impact Factor